1. Field of the Invention
This invention relates to heavy duty pneumatic tires, and more particularly to a heavy duty pneumatic radial tire for use in vehicles such as a dump truck, a loader, a grader and the like.
2. Description of the Related Art
As an approach for the design of the conventional heavy duty pneumatic tire, an outer profile of a tread crown portion is first designed from a viewpoint that distribution of ground contact pressure and distribution of heat build-up over a whole of a tread are made uniform as far as possible. Then a mold for the vulcanization of the tire is designed, and thereafter an internal structure of the tire, to satisfy different requirements in accordance with applications of the tire is designed assuming the use of such a mold.
That is, the conventional heavy duty pneumatic tire is designed from a viewpoint of ground contact pressure distribution and heat build-up distribution as mentioned above. Therefore, as shown in FIG. 1, the tread crown portion has an outer profile that the outer profile line extends from a widthwise center point C of a tread portion toward each side end E thereof up to a shoulder portion at a single and large crown radius R and then connects to a buttress line at the side end E to form a square shoulder or in the vicinity of the side end E through a somewhat small shoulder radius r to form a round shoulder. The crown radius R is frequently determined by calculating a ratio to a section width of the tire or a ratio of fall d of the side end E in the tread to an outer diameter D of the tire from commercially available existing tires.
Although conventional heavy duty pneumatic tires are designed based on the above idea and technique, there is no consideration for designing the outer profile of the tread crown portion from a viewpoint that a belt durability is enhanced by reducing strain produced at an end of a cross belt as far as possible. As a result, in the prior art a limit to enhancement of belt durability exists even if the structure of the cross belt is changed.
In the heavy duty pneumatic radial tire, the cross belt is frequently comprised of at least two main cross belt layers each containing substantially inextensible steel cords, which have a function of bearing circumferential tension, and at least one belt protection layer containing high-extensible steel cords having an elongation at break of 4.5-8%. Even in this cross belt, belt separation is apt to be caused at an end portion between the main cross belt layers due to interlaminar shearing strain and at a free end portion of the belt protection layer due to tensile strain in cord direction.
Particularly, as the tire becomes flatter, the strain produced at the end portion of the belt becomes larger, so that there are frequently observed failures of breaking the belt portion due to belt separation and the like before the service life of the tire itself is lost by complete wearing of the tread rubber. For this end, it is strongly demanded to enhance the belt durability by reducing the strain produced at the end portion of the belt as far as possible. However, when the outer profile of the tread crown portion is already designed, there is a limit for designing the internal structure of the tire to satisfy requirements for the enhancement of the belt durability.